Medical workers in Liberia are set to go on strike over a pay dispute, a move that could deliver a serious blow to the fight against the deadly Ebola virus.

Thousands of members of Liberia's National Health Workers Association said they will not report for work on Monday unless they receive a substantial increase in hazard pay.

Members of the association are demanding $700 in monthly hazard pay on top of monthly salaries that are around $200 or $300.

Liberia has the highest number of infections and deaths in the worst outbreak of the illness. Ebola has killed more than 2,300 people in the poor West African nation.

Obama calls for action

President Barack Obama called for immediate steps to ensure the U.S. medical system is ready to follow the proper protocol for treating Ebola patients.

Obama spoke Sunday with Health and Human Services Secretary Sylvia Burwell about the first case of Ebola transmission in the United States.

Dallas Mayor Mike Rawlings speaks to residents on the street of the apartment of a hospital worker in Dallas, Oct. 12, 2014.

Obama ordered the U.S. Centers for Disease Control and Prevention to quickly investigate how a nurse at Texas Health Presbyterian Hospital contracted the virus.

The nurse was part of the team treating the first Ebola patient to be diagnosed in the United States, Thomas Duncan, who died last week.

Texas officials say the health worker wore full protective gear and complied with CDC guidelines when she had contact with Duncan.

Breach of protocol

CDC Director Tom Frieden said the fact the worker was infected with Ebola shows there was a breach of protocol.

Frieden said, at this time, it appears only one person may have had contact with the health care worker while she may have been infectious. However, the case in Texas indicated a professional lapse that may have caused other health workers at the hospital to also be infected, he added.

Initial testing showed that the level of virus in the worker's system is low. The CDC will conduct a secondary test to confirm the results from a lab in Austin that showed Ebola infection, he said.

“Unfortunately it is possible in the coming days that we will see additional cases of Ebola,” Frieden added.

A union for registered nurses said the Texas health care worker's case in Dallas showed that not enough is being done to educate health workers on how to manage patients who show signs of infection.

“Handing out a piece of paper with a link to the Centers for Disease Control, or telling nurses just to look at the CDC website - as we have heard some hospitals are doing - is not preparedness,” said Bonnie Castillo, a registered nurse and senior official with National Nurses United.

Teresa Romero, the Spanish nurse who contracted Ebola, is seen inside her room at an isolation ward on the sixth floor at Madrid's Carlos III Hospital, Oct. 11, 2014.

Meanwhile, in Spain, authorities said a woman in Madrid infected with Ebola remains in serious condition, but is showing signs of slight improvement and that the level of virus in her blood is decreasing.

US airport screenings

The new U.S. case comes as several airports begin a stepped-up program to screen passengers arriving from Liberia, Sierra Leone and Guinea, where thousands of people have died from the virus.

Authorities at New York's JFK International Airport are taking passengers' temperatures and asking them questions to determine whether they may have come into contact with an infected person.

The enhanced screening will expand to four other airports Thursday, including the Newark airport in New Jersey, Washington's Dulles Airport, Chicago's O'Hare and the international airport in Atlanta.

Together, the five airports receive more than 90 percent of all travelers entering the United States from the three worst-affected West African countries.

Britain is also introducing Ebola screening at certain airports and train stations.

The World Health Organization says the Ebola outbreak in West Africa has killed at least 4,000 people with about 8,400 reported cases.

Outside of Liberia, Sierra Leone and Guinea, eight people died in Nigeria and one patient died in the United States.